Start-up system for forced flow vapor generator and method of operating the vapor generator



Jan. 2, 1968 R. J. BATYKO 3,361,117 STARTUP SYSTEM FOR FORCED FLOW VAPOR GENERATOR AND METHOD OF OPERATING THE VAPOR I GENERATOR Filed Feb. 18, 1966 I NVEN TOR.

Roberf J. Baryko BY ATTO RNEY United States Patent 3,361,117 START-UP SYSTEM FOR FORCED FLOW VAPOR GENERATOR AND METHOD OF OPERATING THE VAPOR GENERATOR Robert J. Batyko, Wadsworth, Ohio, assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Feb. 18, 1966, Ser. No. 528,523 5 Claims. (Cl. 122406) ABSTRACT OF THE DISCLOSURE A system for starting a power plant containing a turbine, a forced flow boiler having a through flow circuit including a vapor generating section, a first superheater and a second superheater connected for series flow of fluid in the order named, and auxiliary equipment including a deaerator and a condenser. Startup of the power plant includes directing a vaporiza'ble fluid at about full load pressure through the vapor generating section in indirect heat transfer relation with heating gases and at a rate less than full load flow. Portions of the fluid so heated are reduced in pressure and passed directly to the condenser, to the deaerator and to the first superheater. First superheater outflow is reduced in pressure and passed directly to the deaerator. Fluid is recirculated through the vapor generating section, first superheater, and auxiliary equipment back to the vapor generating section to increase the enthalpy of the first superheater outflow to a predetermined value. Then equilibrium of fluid pressure between the first and second superheaters is established by directing first superheater outflow through the second superheater to the turbine to warm and roll the turbine.

This invention relates in general to power plant system having a turbine arranged to be supplied with vapor from a forced circulation vapor generator and more particularly to apparatus for and a method of starting up such a system.

The general object of the present invention is the provision of a starting system of the character described so constructed and arranged as to simplify starting procedure; to provide low cost, rapid, controlled start-ups; to provide adequate protection of the turbine and vapor superheating section of the vapor generator to the end that thermal stresses on this equipment are within acceptable limits at all times; to provide heat recovery during starting up and low load operation; and to permit matching of the vapor temperature to turbine metal temperature on hot restarts and a minimum diiierential of vapor temperature and turbine metal temperature on cold starts.

l-leretofore, it has been proposed to place a shut-off valve between the primary and secondary superheaters with a flash drum in circuitry bypassing the shut-off valve. The separation of the superheaters by use of the valve was necessary so that each could be started independently. The flash drum arrangement, however, is disadvantageous due to the high cost of the heavy drum plate required, the safety precautions that must be taken with a large vessel operating at high pressure, the many controls required for the drum and the relatively large size of the drum which substantially increases the overall size and cost of the vapor generator. The present invention avoids these disadvantages by providing a simple and relatively inexpensive system which permits start-up or shut-down of the vapor generator without the use of a flash drum, provides for by-passing vapor leaving the 3,361,117 Patented Jan. 2, 1968 vapor generator until vapor conditions are satisfactory for admission to the turbine, and provides saturated vapor at the high temperature superheater inlet for turbine warming and rolling.

As generators of the forced flow type are well-known in the art, I have shown in the drawing the elements of such a generator in rudimentary form to assist in understanding my invention. While the starting system of the invention is adapted for use in a forced flow vapor generating and superheating unit designed for the production of superheated vapor at pressures and temperatures below the critical pressure of 3206 psi. and the critical temperature of 705 F., a unit of this general construction being described and claimed in US. Patent No. 3,125,995, issued Mar. 24, 1964, it will be understood that the invention starting system can also be advantageously used in a forced flow vapor generator designed for supercritical pressures and temperatures.

In the power plant system illustrated, feedwater is supplied by a boiler feed pump 10 through a conduit 11 to a high pressure feedwater heater 12, then passes through a conduit 13 to the circuitry of a vapor generator 14 of the once-through forced flow type having a series fluid flow path including an economizer 16, vapor generating section 17, and superheating section 18. Superheating section 18 comprises a primary superheater 18A, connected by a conduit 19 for flow of fluid from vapor generating section 17, and a secondary superheater 1813, connected for series flow of fluid from primary superheater 18A by a conduit 15 containing a stop valve 20 and for series flow of fluid to a high pressure turbine 22 by a conduit 23 containing a stop valve 24 and a control valve 26. Valve 24 is by-passed by a conduit 21 containing a stop valve 25. Fuel and air for combustion are admitted to the vapor generator 14 through conduits 27 and 2-8 respectively; and the gaseous products of combustion, after passing over the vapor generating and superheating surfaces and economizer, are discharged through an outlet, diagrammatically shown at 29. Valves 27A and 28A represent the customary regulating means for the fuel and air respectively.

During normal operation, the working medium passes successively through economizer 16, vapor generating section 17 and superheating section 18. The superheated vapor outflow of superheating section 18 passes through conduit 23 to the high pressure stage of vapor turbine 22 for expansion therein, with the vapor then exhausting through a conduit 30 to a main condenser 31, Where it is condensed at a low pressure for return to the feedwater system. From the condenser the condensate passes by way of a conduit 32 to a pump 33 from which it discharges through a conduit 34 and low pressure feedwater heater 36 to a direct contact type deaerating heater 37 which serves to boil the condensate to eliminate any entrained oxygen. Condensate from the deaerator passes through a conduit 38 to the suction side of feed pump 10 for return to the vapor generator.

In accordance with the invention, a special by-pass system is provided around the superheaters 18A and 18B and around the turbine. This system is constructed and arranged to obtain highest operational flexibility, to reduce heat losses, and to provide full thermal protection of superheater surface and the turbine; and is used for cold and hot startups, for low load operation and for shutdown or emergency trip of the vapor generator.

The primary superheater by-pass comprises a conduit 39 having branches 54, 57 and 62, with branch 54 leading to heater 12 and containing a pressure breakdown valve 56, branch 57 leading to deaerator 37 and containing a pressure breakdown valve 53 and branch 62 leading to the condenser 31 and containing a pressure breakdown valve 63, spray attemperator 64, and atte-mperator control valve 64A. Attemperator 64 is provided to reduce fluid temperature to conditions acceptable to the condenser. The secondary superheater by-pass includes a conduit 55 containing a pressure breakdown valve 41 and having one end connected and opening to conduit at a position upstream of valve and its opposite end connected and opening to deaerator 37. Valve 20 is bypassed by a conduit 15A containing a pressure reducing valve 20A and having one end opening and connected to conduit 55 at a point upstream of valve 41 and its opposite end connected and opening to conduit 15 at a location downstream of valve 20. The connections to the high pressure feedwater heater and the deaerator are used to recover heat for the cycle during start-up and the connection to the condenser is used during water cleanup or to handle the excess flow beyond that required by the deaerator and heater.

In a typical cold start-up of the power plant system illustrated, about one-quarter to one-third full load water flow is established through the boiler feed pump 10 which takes suction from deaerator 37 and forces fluid successively through conduit 11, heater 12, conduit 13, economizer 16, vapor generating section 17, conduit 39, and conduit 62 to the condenser. superheater stop valve 20, pressure reducing valve 20A, secondary superheater bypass valve 41, turbine stop valve 24, and turbine stop valve by-pass are closed so that there is no flow through superheating section 18. Valve 63 is set to maintain full load pressure at the outlet of the vapor generating section throughout the start-up. Valves 56 and 58 are also closed causing all flow initially to go to the condenser. The turbine is sealed by steam from an auxiliary source and condenser vacuum is pulled. Firing is then commenced and the gas temperature entering superheater 18 is held to approximately 1000 F. As the enthalpy of the fluid entering conduit 39 increases valve 58 is opened allowing flow to the deaerator, causing deaerator pressure to rise. When the pressure in the deaerator 37 reaches a value equal to its normal pressure at onequarter of full load, which is about 20 p.s.i.g., valve 53 begins to close in order to hold the pressure at this point. When the deaerator pressure is at its desired point, valve 56 opens to allow flow to the high pressure heater 12 and thereby recover more heat for the cycle. As the enthalpy of the fluid in conduit 39 continues to increase, the pressure in heater 12 increases until it reaches a set point equal to the heater full load operating pressure, which is around 500 psig At that point valve 56 will regulate the flow to the heater to maintain the pressure set point. Drain flow from the heater passes through conduit 71 to the condenser, with valve 72 being used to control water level in the heater.

While outflow of vapor generating section 17 is directed through conduit 39 to deaerator 37 and heater 12 in the initial phase of start-up, secondary superheater by-pass valve 41 is kept closed until the water temperature in conduit 39 exceeds 300 F. By so controlling valve 41, it is never subjected to the severely erosive service of pressure breakdown of cold water. When the water temperature in conduit 39 exceeds 300 F., valve 41 is opened to a predetermined minimum position so that a portion of the vapor generating section outflow is directed through primary superheater 18A and then through conduit 55 for flow to deaerator 37, while another portion of vapor generating section outflow is directed to the deaerator and heater 12 by way of conduits 57 and 54, respectively. Valve 41 is maintained in this minimum position until the valve inlet fluid temperature is such that its corresponding enthalpy is about 1200 B.t.u./lb. After the fluid entering valve 41 is so conditioned, the valve opening is regulated to maintain such set point temperature, while valve 63 is regulated to hold full load pressure at the outlet of the primary superheater throughout the start-up. By way of example, the set point temperature of valve 41 is 780 F. for a full load operating pressure of 3500 p.s.i.g. in primary superheater 18A and 705 F. for a full load operating pressure of 2400 p.s.i.g. in primary superheater 18A.

The passage of working fluid through primary superheater 18A and conduit 55 to deaerator 37 results in the heat absorbed by the primary superheater passing to the deaerator. In prior systems, primary superheater outflow was successively directed through the secondary superheater and a turbine by-pass to the condenser so that primary superheater heat absorption did not get to the deaerator. By directing the heat absorption of the primary superheater to the deaerator where it can be recovered, the unit may be heated up faster with a given firing rate or heated in the same time with a lower firing rate. Either way there is a saving in total fuel expended for start-up compared to prior systems.

When the flow through primary superheater 18A reaches a predetermined rate as determined by the amount of opening of valve 41, then valve 20A is opened slightly to allow flow through conduit 15A, secondary superheater 18B and main steam conduit 23 to a valve controlled drain line 66, thereby warming the main steam conduit and the secondary superheater. Since the enthalpy of the fluid entering valve 41A is maintained constant at about 1200 B.t.u./lb., the fluid passing through valve 20A to the secondary superheater is steam in a dry condition. Drain line 66 leads from above the seat of stop valve 25 and its discharge may be directed to condenser 31. During opening of valve 20A, valve 41 is controlled to close an equal amount so that there is no flow unbalance in primary superheater 18A.

As firing continues sufiicient flow becomes available through valve 20A to roll the turbine. Thereupon flow through drain line 66 is discontinued. Steam flow to the turbine is controlled with turbine stop valve 25, allowing steam admission to the turbine with valves 26 wide open, which results in uniform heating of valve bowls, valve chests, and turbine first stage shell area. Valve 25 is controlled to throttle the pressure of the steam down to a level sufficient to roll the turbine. Any excess fluid from the primary superheater is discharged to the deaerator by way of conduit 55. The amount of opening of valve 41 serves as an index of the quantity of fluid available for increasing turbine speed or load.

Since the heat absorbed by primary superheater 18A during start-up serves to generate steam, much lower firing rates are required than on prior start-up systems where it served to superheat steam. The start-up system of the invention is particularly adapted for use in a forced flow vapor generating and superheating unit of the type described in US. Patent No. 3,125,995 wherein the primary superheater is disposed in the convection gas pass and absorbs heat primary by convection and the secondary superheater is located in the convection gas pass upstream gas-wise of the primary superheater. A primary superheater so arranged is particularly effective in absorbing heat during start-up when high excess air, high flue gas tempering or high flue gas recirculation is used. In addition, the low firing rates result in low gas temperature at the secondary superheater and therefore low steam temperatures for rolling a cold turbine. When the startup system is used to start a hot turbine, the unit may be fired at a higher rate to obtain high steam temperatures for rolling the turbine, and conduit 62 will relieve excess steam to condenser 31.

At a predetermined point in the loading of the turbine, control of the turbine is transferred from valve 25 to valves 26. Thus when the turbine is loaded to approximately 8% of maximum continuous rating, it is normally changed to partial arc-admission by simultaneously opening valve 25 and closing valves 26. Now valve 24 can be opened wide and valve 25 closed, From this point on, it is desirable to further load the turbine with throttle valves 26 in a fixed position to maintain the throttling temperature drop across valves 26 nearly constant. Load secondary superheater must be further pressurized by further opening valve 20A, thereby increasing the flow to the turbine. Control valves 26 are in a position corresp nding to approximately 30% of full load. Since, at this time, valve 20A is handling all vapor at a high specific volume, and since the outflow of valve 20A is directed to the relatively small volume of secondary superheater 188, the secondary superheater pressure increase in response to such valve opening is almost immediate. This pressure response can be used as an index to correct the position of the pressure reducing valve 20A. The result is an orderly and controlled pressurization of the secondary superheater 18B since firing rate, secondary superheater outflow, and the position of valve 20A can be closely paralleled. As valve 20A is gradually opened, primary superheater outflow to the deaerator is gradually decreased by regulation of valve 41 so that flow through the primary superheater remains substantially constant until primary superheater outflow to the deaerator is cut off. Throughout start-up the primary superheater 18A is at a constant pressure, so that its heat storage does not change. During pressurization of the secondary superheater and loading of the turbine, the primary superheater outlet temperature set point, which governs the setting of secondary superheater bypass valve 41, can be adjusted to follow the natural heat absorption characteristics of the primary superheater. In application, valve 41 will be fully closed at about full turbine load above which the primary superheater becomes increasingly effective in superheating steam, while the generation of steam therein gradually decreases.

When valve A is wide open, valves 41, 56, 58 and 63 are closed and all flow goes directly to the superheater. Normal turbine extraction flows will be used for deaeration and feedwater heating. At this time, the high pres sure superheater stop valve 20 can be opened wide and valve 20A closed.

The operation sequence for hot restarts and intermediate starts is essentially the same as for a cold start, except that the firing rate is higher in order to obtain higher steam temperatures leaving the secondary superheater.

While in accordance with the provisions of the statutes I have illustrated and described herein the best form and mode of the invention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by the claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

I claim:

1. In a power plant including a turbine, a forced flow boiler having a through flow circuit including a fluid heating section, a first superheater and a second superheater connected for series flow of fluid in the order named, and auxiliary equipment including a deaerator and a condenser, the method of starting up the power plant comprising passing a vaporizable fluid at about full load pressure through the fluid section in indirect heat transfer relation with heating gases and at a rate substantially less than full load flow,

passing portions of the fluid so heated directly to the condenser and to the deaerator, while maintaining full load pressure in the fluid heating section and while reducing fluid pressure and temperature to conditions suitable for introduction to the condenser and deaerator,

passing another portion of the fluid heating section outflow through the first superheater in indirect heat transfer relation with the heating gases, passing first superheater outflow directly to the deaerator, while reducing its pressure and temperature to conditions suitable for introduction to the deaerator,

recirculating fluid through the fluid heating section, first superheater, and auxiliary equipment to the fluid heating section to increase the enthalpy of the first superheater outflow to a predetermined value, while maintaining pressure in the fluid heating section and the first superheater at about full load pressure,

gradually establishing equilibrium of fluid pressures between the first and second superheaters by directing and gradually increasing first superheater outflow through the second superheater to the turbine to warm and roll the turbine, while gradually decreasing first superheater outflow to the deaeator so that flow through the first superheater remains substantially constant until first superheater outflow to the deaerator is cut off.

2. The method of claim 1 wherein the enthalpy of the first superheater outflow to the second superheater is maintained at a value at least corresponding to said predetermined value While establishing equilibrium of fluid pressures between the first and second superheaters, with this value being such as to assure that the fluid is in a substantially dry condition in flowing to the second superheater.

3. The method of claim 1 wherein the auxiliary equipment includes a heat exchanger having a primary side and a secondary side, and a portion of the fluid heating section outflow is passed directly to the secondary side of the heat exchanger while its pressure and temperature are reduced to conditions suitable for introduction to the heat exchanger.

4. In a power plant in which, during normal operation, a vaporizable fluid is successively passed through a fluid heating section, a first superheater and a second superheater to a vapor turbine, a start-up system comprising means for generating heating gases, means for passing vaporizable fluid at about full load pressure through the fluid heating section in indirect heat transfer relation with the heating gases and at a rate substantially less than full load flow,

auxiliary equipment including a deaerator and a condenser, means for passing portions of the fluid heating section outflow directly to the condenser and to the deaerator, while reducing fluid pressure and temperature to conditions suitable for introduction to the condenser and deaerator, means for passing another portion of the fluid heating section outflow through the first superheater in indirect heat transfer relation with the heating gases,

means for passing first superheater outflow directly to the deaerator, while reducing its pressure and temperature to conditions suitable for introduction to the deaerator,

means for recirculating fluid through the fluid heating section, first superheater, and auxiliary equipment to the fluid heating section to increase the enthalpy of the first superheater outflow to a predetermined value, while maintaining pressure in the fluid heating section and the first superheater at about full load pressure,

means for directing and gradually increasing first superheater outflow through the second superheater to the turbine to establish equilibrium of fluid pressures in the first and second superheaters and to warm and roll the turbine, and

means for maintaining the enthalpy of the first superheater outflow at about said predetermined value until equilibrium of pressures between the first and second superheaters is established.

5. A start-up system for a power plant as defined in claim 4 wherein said auxiliary equipment includes a heat exchanger having a primary side and a secondary side, means are provided for passing a portion of the fluid heating section outflow directly to the secondary side of the heat exchanger while reducing fluid pressure and temperature to conditions suitable for introduction to the heat exchanger.

References Cited UNITED STATES PATENTS KENNETH W. SPRAGUE, Primary Examiner. 

